The invention relates to multi-station strip metal forming presses of the type including a vertically reciprocating ram for mounting and operating the tools. These presses are well suited for the manufacture of metal parts made sequentially in a succession of drawing operations at the stations. In this type of machine, coil strip stock is fed in widths from a fraction of an inch through up to seven inches on the larger machines. Blanks are automatically cut from the strip, and vertical blank transfer mechanism positively holds and carries the blank down to the transfer level, where it is picked up by the transfer fingers of a horizontal transfer mechanism. Usually the blank is transferred through a succession of draw dies in as many as fifteen individual work stations and, finally, ejected as a completed part. This type of completely automatic operation allows piercing, forming, drawing, lettering, embossing and flanging, as well as side slotting, side piercing and reverse drawing at production rates which have exceeded 250 parts per minute. From blanking operation to finished part ejection, all tooling is mounted in standardized precision die sets to facilitate set up and minimized down time. Each station may be individually adjusted or serviced. Complete die sets can be interchanged without losing tool adjustment. Frequent complete change of jobs or intricate toolings will justify extra die sets.
An example of an early press of this type developed by the assignee of the instant application which sets forth the essential nature of this type of press is found in U.S. Pat. No. 2,049,915 dated Aug. 4, 1936, in the name of Arthur J. Lewis and assigned to the assignee of the instant application. Of course, a large number of improvements have been made since the issuance of the Lewis patent; primarily with a view toward producing higher speeds, lower ultimate tooling costs, precision operation, tool adjustment and replacement, minimizing down time, minimizing scrap loss and, in general, providing greater versatility and operational sophistication for the presses.
The type of press to which my invention pertains is subject to having jams occur, as by work pieces breaking or being improperly positioned at or between work stations during operation, with the attendant risk of seriously damaging expensive components of the press. Some attempts to avoid or minimize such damage have been made by those skilled in the art, as by incorporation of slip clutches in the drive means for the transfer mechanism; however, with only modest success, because slip clutches inherently do not provide "linear" protection throughout the full cycle of operation of the transfer mechanism drive means, i.e., they do not respond to a fixed uniform overload, but rather varying overloads depending upon the cam angle at the time of overload, because the usual drive means is inherently nonlinear.
It is the purpose of my invention to provide an improved transfer mechanism for a multi-station transfer press having a safety release means incorporated in it which reliably provides linear protection; i.e., against jams occurring at any point in the cycle of operation or at any position in the press in response to a fixed, uniform level of overload.
Presses of the type to which my invention pertains usually are provided either without overload protection or with a conventional slip clutch on a drive shaft for the transfer mechanism. Usually the transfer slide is reciprocated horizontally by a cam mounted on a vertical side shaft. The action or motion of the transfer slide is at right angle to the axis of the side shaft. The transfer slide is driven by a conjugate cam mounted on the side shaft which runs against two cam rollers mounted at the top surface of the transfer slide. With each cam revolution, the transfer slide moves to the right, dwells, then moves to the left and dwells again.
Conventionally with the standard unprotected system, when something occurs in the machine or tooling, which accidentally stops the usual transfer rails associated with the transfer slide from moving, then the much more substantial transfer slide driven by the powerful cam continues to try to move the transfer rails. The torque and strength of the side shaft is very great and the cam and transfer slide are very strong. Therefore, when the transfer rails are jammed and stop, something has to give or break. Frequently, it is an expensive press component.
Conventional slip clutch, overload release action, when a slip clutch is provided, results in overload release forces completely dependent on the particular location of the transfer slide when the jam occurs. The conventionally used constant torque release slip clutch is usually mounted on the side shaft and it drives a cam that causes exceptionally high transfer slide forces at the beginning and end of the drive motion and very low forces during the rapid motion of the cam near the middle of the transfer slide stroke. Conventionally, a slip clutch is attached to the drive cam and expected to disengage and release the drive connection when the transfer slide jams, but the force on the transfer slide and cam required to cause disengagement at the beginning and end of motion are much too great with that type of release protection to make the clutch release at a sufficiently lower overload force. This exceptionally high transfer slide load at the start and end of motion, where the cam angle is high, contrasts with too small an overload force in the center of motion required to pop out the conventional release slip clutch, and results in clutch release at too low an overload force causing nuisance releases.
There is a need for my invention, because there is no way of knowing at what position in the press a jam will occur or at what point the transfer slide will be stopped by a jam during its motion. It is imperative that the protective safety release means inactivate the drive at some previously selected uniform overload, and that the release take place at that amount of overload regardless of the location of the jam or operational point during the slide motion. This desirable action is referred to as "linear" protection; a slip clutch does not provide it.